The present invention relates to sintered phosphate of calcium ceramics having a high strength and density which are useful as bioceramics replaceable for bones or teeth and as dense ceramic substrates, e.g., for IC package. Phosphate of calcium, especially hydroxyapatite is bio-compatible and the sintered ceramics thereof have been proposed to be used as replacements for bones and teeth, i.e., as bio-ceramics. Due to the high density, phosphate of calcium ceramics are also useful as a substrate for IC package and a ceramic material having a high expansion coefficient similar to metal.
In the known manners for manufacturing phosphate of calcium ceramics, there have been a conventional sintering process wherein the pressed body is sintered under the atmospheric pressure, hot-press process and so on. When powdery hydroxyapatite as phosphate of calcium is sintered by said normal conventional sintering process the resultant product shows a compression strength of about 900 kg/cm.sup.2 (Ceramics Japan Vol. 10 [7] 1975, P474), a low density and weak resistability against spalling, i.e. tends to spall in the course of cooling after sintering, which are counted as drawbacks of this process.
On the other hand, a compression strength of approximately 1000-2000 kg/cm.sup.2 may be obtained by the hot-press process (Reports of the Institute for Medical & Dental Engineering Vol. 7, P113-118, 1973). However, this process is quite complicated, requiring an expensive apparatus cost and not susceptible of mass-production, which make a drawback of a high producing cost as a whole. In this connection the compression strength required as bone is about 4,000 kg/cm.sup.2, generally.
U.S. Pat. No. 4,135,935 Pfeil et al discloses a process for sintered product wherein 20 parts by weight of precipitated fluorapatite and 34 parts by weight of SiO.sub.2 -P.sub.2 O.sub.5 -Na.sub.2 O-K.sub.2 O-MgO-CaO glass composion is pressed, then sintered at 670.degree. C. This glass composition comprises by weight 20-60% of SiO.sub.2, 5-40% of P.sub.2 O.sub.5, 2.7-20% of Na.sub.2 O, 0.4-20% of K.sub.2 O, 2.9-30% of MgO, and 5-40% of CaO. This sintered product comprises only 37% by weight of apatite, and the balance is said glass composition.